In a photosensitive unit equipped in an image forming apparatus such as a laser printer, a driving force from an image forming apparatus body is transmitted to a drum body of a photosensitive drum which receives laser from an exposing unit and is formed with an electrostatic latent image, through the driving force transmission mechanism such as a gear disposed at the axial end portion of the drum body. Additionally, a developing roller for supplying toner to the drum body is disposed so as to be in press contact with the drum body, and a driving force from the image forming apparatus body is transmitted to the developing roller through the driving force transmission mechanism disposed at the axial end portion thereof. Thereafter, the driving force is transmitted to rotate the drum body and the developing roller, the toner is fed to the electrostatic latent image on the drum body to form a toner image on the drum body, and the toner image is then transferred onto the sheet.
In such image forming apparatus, the rotational speed of the developing roller is generally set higher than the rotational speed of the drum body for smoothly supplying toner to the drum body. Therefore, the rotational torque that corresponds to the difference between the rotational speeds acts on the drum body, which may increase the rotational speed of the drum body. Additionally, when a gear is employed as the driving force transmission mechanism described above, a backlash from the gear may change the rotational speed of the drum body.
Such problems may cause a shift in the timing of receiving laser beam and the timing of transferring the toner image from the drum body onto the sheet, in the drum body, resulting in an image shift in the rotational direction of the drum body during image formation.
A structure is therefore proposed to maintain the constant-speed rotation of the drum body by controlling a change in the rotational speed of the drum body.
As a structure for maintaining the constant-speed rotation of the drum body, for example, a structure having a frame inside the image forming apparatus, a photosensitive body and a dumping mechanism is known. The photosensitive body is rotatably connected to the frame via the rotation shaft thereof. A dumping mechanism includes a pressurizing member through which the rotation shaft of the photosensitive body extends and which has a friction pad attached on a surface opposed to a flange at the end portion of the photosensitive body; and a coil spring through which the rotation shaft of the photosensitive body extends and which urges the pressurizing member toward the flange of the photosensitive body. With this structure, elastic pressure of the friction pad is applied to the flange at the end portion of the photosensitive body, thereby giving a frictional force to the rotation of the photosensitive body and restricting the above described changes in the speed.
A structure for maintaining constant-speed rotation has been proposed. In this structure, a damper having a core through which the rotation shaft of the photosensitive body extends and a pair of wings extending from the external peripheral portion of the core, is placed in a recessed groove portion formed between a first circular projection portion and a second circular projection portion provided in the flange of the photosensitive body, and the wings of the dumper contact the internal surface of the first circular projection portion of the photosensitive body during the rotation of the photosensitive body to apply frictional force to the rotation of the photosensitive body.
However, there is a limit in securing a contact area of the flange and the pressurizing member since the coil spring urges the pressurizing member toward the flange along the axial direction of the photosensitive body and the contact surface of the friction pad of the pressurizing member and the flange are orthogonal to the urging direction of the coil spring (axial direction of the photosensitive body). This may also limit the magnitude of the frictional force applied to the rotation of the photosensitive body.
On the other hand, in the latter structure for maintaining the constant-speed rotation, as a rotation prevention unit to prevent the simultaneous rotation of the dumper and the flange, it is necessary to form a stopper in the dumper and form a stopper accommodation groove for accommodating the stopper in the frame, leading to a complex structure.
In conventional constant speed rotation maintaining structures including the above-described structures, although some effects have been recognized for maintaining the constant speed rotation in black and white laser printers that have one photosensitive body, it may not be able to maintain an adequate level of the constant speed rotation in color laser printers that have a plurality of photosensitive bodies, since an image shift may be caused due to a shift in the transferring timing onto the sheet in the case where the printer even slightly fails to maintain the constant speed rotation between the photosensitive bodies.
Further, in an image forming apparatus in which a rotation shaft is provided unrotatably with respect to the frame and a photosensitive body is provided rotatably with respect to the rotation shaft, a slight gap (50 μm in diametric difference, for example) is uniformly secured in the radial direction on the periphery of the rotation shaft between the photosensitive body and the rotation shaft since the photosensitive body slides with respect to the rotation shaft. Therefore, if a frictional force generated in the constant speed rotation maintaining structure does not act uniformly upon the rotational direction with respect to the rotation of the photosensitive body, the gap between the photosensitive body and the rotation shaft will not be uniform on the periphery of the rotation shaft and thus the rotation of the photosensitive body is decentered. The decentered rotation of the photosensitive body leads to an image shift caused by a change of the photosensitive body in the radial direction with respect to the rotation shaft.